Fix Mac App Store Error 100

“We could not complete your App Store request. An unknown error occured (100). There was an error in the App Store. Please try again later.”

Hmm, well trying again later doesn’t solve anything, but this does:

Fix the Mac App Store “Unknown Error 100″

Sign out of the Mac App Store via the Store menu

Quit the Mac App Store

Log out of your Mac OS X user account (from the Apple menu)

Log back in to your user account

Launch the Mac App Store, sign in, and try downloading again

You should be able to download any app trouble free now. If you still have problems, reboot your Mac.

I originally resolved the problem by rebooting (the default troubleshooting tip when all else fails) but MacStories found that just logging out of the user account and back in is enough to make the App Store work as intended.